Think the ol' ab wheel is tough? Try these more advanced variations.

Foam Roller Rollouts

Use these two exercises as a progression from the ab wheel. The videos don't do them justice. You've got to try them to understand it.

Keep these in a low to moderate rep range, multiple sets of 5-8 reps. Going high rep on them can make the low back kick in. But you've been warned: This will leave even veteran lifters shaking like a leaf on a tree.